The present invention relates to a solid colored composition, which in some embodiments may be employed as an electrophotographic toner, e.g., a toner employed in a photocopier which is based on transfer xerography.
Electrophotography is broadly defined as a process in which photons are captured to create an electrical image analogue of the original. The electrical analogue in turn is manipulated through a number of steps which result in a physical image. The most commonly used form of electrophotography presently in use is called transfer xerography. Although first demonstrated by C. Carlson in 1938, the process was slow to gain acceptance. Today, however, transfer xerography is the foundation of a multibillion dollar industry.
The heart of the process is a photoreceptor, usually the moving element of the process, which is typically either drum-shaped or a continuous, seamless belt. A corona discharge device deposits gas ions on the photoreceptor surface. The ions provide a uniform electric field across the photoreceptor and a uniform charge layer on its surface. An image of an illuminated original is projected through a lens system and focused on the photoreceptor. Light striking the charged photoreceptor surface results in increased conductivity across the photoreceptor with the concomitant neutralization of surface charges. Unilluminated regions of the photoreceptor surface retain their charges. The resulting pattern of surface charges is the latent electrostatic image.
A thermoplastic pigmented powder or toner, the particles of which bear a charge opposite to the surface charges on the photoreceptor, is brought close to the photoreceptor, thereby permitting toner particles to be attracted to the charged regions on the photorecepor surface. The result is a physical image on the photoreceptor surface consisting of electrostatically held toner particles.
A sheet of plain paper is brought into physical contact with the toner-bearing photoreceptor. A charge applied to the back side of the paper induces the attraction of the toner image to the paper. The image is a positive image of the original. The paper then is stripped from the photoreceptor, with the toner image clinging to it by electrostatic attraction. The toner image is permanently fused to the paper by an appropriate heating means, such as a hot pressure roll or a radiant heater.
Because there is incomplete transfer of toner to the paper, it is necessary to clean the photoreceptor surface of residual toner. Such toner is wiped off with a brush, cloth, or blade. A corona discharge or reverse polarity aids in the removal of toner. A uniform light source then floods the photoreceptor to neutralize any residual charges from the previous image cycle, erasing the previous electrostatic image completely and conditioning the photoreceptor surface for another cycle.
The toner generally consists of 1-15 micrometer average diameter particles of a thermoplastic powder. Black toner typically contains 5-10 percent by weight of carbon black particles of less than 1 micrometer dispersed in the thermoplastic powder. For toners employed in color xerography, the carbon black may be replaced with cyan, magenta, or yellow pigments. The concentration and dispersion of the pigment must be adjusted to impart a conductivity to the toner which is appropriate for the development system. For most development processes, the toner is required to retain for extended periods of time the charge applied by contact electrification. The thermoplastic employed in the toner in general is selected on the basis of its melting behavior. The thermoplastic must melt over a relatively narrow temperature range, yet be stable during storage and able to withstand the vigorous agitation which occurs in xerographic development chambers.
The success of electrophotography, and transfer xerography in particular, no doubt is a significant factor in the efficient distribution of information which has become essential in a global setting. It also contributes to the generation of mountains of paper which ultimately must either be disposed of or recycled. While paper is recycled, it presently is converted to pulp and treated to remove ink, toner, and other colored materials, i.e., deinked, an expensive and not always completely successful operation. Moreover, deinking results in a sludge which typically is disposed of in a landfill. The resulting deinked pulp then is used, often with the addition of at least some virgin pulp, to form paper, cardboard, cellulosic packaging materials, and the like.
The simplest form of recycling, however, is to reuse the paper intact, thus eliminating the need to repulp. To this end, toners for copier machines have been reported which are rendered colorless on exposure to near infrared or infrared radiation. Although the spectrum of sunlight ends at about 375 nanometers, it has a significant infrared component. Hence, such toners have a salient disadvantage in that they are transitory in the presence of such environmental factors as sunlight and heat; that is, such toners become colorless. This result is unsatisfactory because the documents can be rendered illegible before their function or purpose has ended. Accordingly, there is a need for toners for copy machines which will permit the recycling of paper intact, but which are stable to normally encountered environmental factors.